Recently, various developments have been made about structures of surface carted magnetic fine particles which are intended to be used for an MRI imaging agent or a thermal source of hyperthermia therapy. JP 2009-114066 (patent literature 1) discloses the method comprising the steps of adding an iron ion solution (ferrous and ferric irons) to an aqueous solution of dextran as a coating material, adding an aqueous solution of sodium hydroxide and then heating and refluxing to prepare dextran magnetite.
FIG. 11 shows a schematic reaction mechanism in the preparation method of the patent literature 1. In the conventional method described in the patent literature 1, the iron ions in the reaction system precipitate to form the magnetite fine particles 50 by adding sodium hydroxide (alkaline) and these immediately coagulate as shown in FIG. 11. In the conventional methods, the dextran molecule 52 coats the coagulants and particle sizes of final products become as large as several tens nm such that there were limitations on improvements in making fine size particles and also in improving dispersibility. In addition, according to the conventional method, the coagulation occurred unevenly and hence it was difficult to control particle sizes evenly.
On the other hand, though the introduction of various biomolecules on the surface on the surface coated magnetic fine particles for biological applications, additional surface modification processing must be required in order to introduce functional groups on the surface of the magnetic fine particles in the case of the conventional surface coated magnetic fine particles.